FIG. 1 illustrates one type of prior art current mode DC/DC switching power supply, also known as a peak current mode DC/DC converter. The converter is a buck converter since the output voltage Vout is lower than the input voltage Vin. Many other converter types, such as a switching voltage mode converter, can also benefit from the present invention.
The operation of the converter is conventional and is as follows.
A clock (CLK) signal is applied to the set input of an RS flip-flop 20.
The setting of the RS flip-flop 20 generates a high signal at its Q output. A logic circuit 24, in response, turns the transistor switch 26 on and turns the synchronous rectifier switch 28 off. Both switches may be MOSFETs or other types of transistors. A diode may replace the synchronous rectifier switch 28. The logic circuit 24 ensures that there is no cross-conduction of switches 26 and 28. The input voltage Vin applied to an inductor L1 through the switch 26 causes a ramping current to flow through the inductor L1, and this current flows through a low value sense resistor 32. The sense resistor 32 may instead be located on the other side of the inductor L1. There are various other ways to detect the inductor current. The ramping current is filtered by an output capacitor Cout and supplies current to the load 38. The output capacitor Cout is relatively large to smooth out ripple.
The output voltage Vout is applied to a voltage divider 42, and the divided voltage is applied to the inverting input of a transconductance error amplifier 44. Capacitors may be connected across the resistors in the divider 42 to further compensate the feedback loop. A reference voltage Vref is applied to the non-inverting input of the amplifier 44. The output current of the amplifier 44 corresponds to the difference between the actual output voltage Vout and the desired output voltage. The voltage (a control voltage Vc) at a capacitor 46, connected to the output of the amplifier 44, is adjusted up or down based on the positive or negative current output of the amplifier 44. The RC time constant of the capacitor 46 and resistor 47 can be adjusted to compensate the feedback loop to improve stability. The transconductance (gm) of the error amplifier 44 can also be adjusted to improve stability. The control voltage Vc, among other things, sets the duty cycle of the switch 26, and the level of the control voltage Vc is that needed to equalize the inputs into the amplifier 44.
The control voltage Vc is applied to a comparator 50. The ramping voltage drop across the sense resistor 32, when the switch 26 is on, is sensed by a differential amplifier 52, which outputs the voltage Visense proportional to the inductor current. When the voltage Visense exceeds the control voltage Vc, the comparator 50 is tripped to output a reset pulse to the RS flip-flop 20. This turns the switch 26 off and turns the synchronous rectifier switch 28 on to discharge the inductor L1, causing a downward ramping current. In this way, the peak current through the inductor L1 for each cycle is regulated to generate a desired output voltage Vout. The current through the sense resistor 32 includes a DC component (the lower frequency, average current) and an AC component (the higher frequency, ripple current).
FIG. 2 shows an example of the ramping voltage Visense (or the inductor current). The DC load current is the average of the triangular waveform.
In some systems powered by the buck converter, it is vital to maintain a reliable output voltage. The capacitance of the output capacitor Cout typically reduces with age, stresses, and temperature variations. This is especially true when the buck converter is powering high current equipment, such as servers and motors. When the output capacitor capacitance reduces, the ripple in the output voltage may exceed a desired amount. Further, when the capacitance reduces, it may result in large perturbations in the output voltage during load transients, which may not be acceptable for certain loads. Such poor regulation can cause instability and indicate failure of the output capacitor.
What is needed is a technique for use in a switching converter that automatically detects the real time value of the output capacitor. Such information may be used to identify an output capacitor failure or to automatically adjust the compensation of the feedback loop to improve stability.